如果你没有设置一个，就没有一个。这个codex 有一个很好的例子来说明如何做到这一点。

<?php
@ini_set(\'log_errors\',\'On\');
@ini_set(\'display_errors\',\'Off\');
@ini_set(\'error_log\',\'/home/example.com/logs/php_error.log\');

/**
 * This will log all errors notices and warnings to a file called debug.log in
 * wp-content (if Apache does not have write permission, you may need to create
 * the file first and set the appropriate permissions (i.e. use 666) ) 
 */
define(\'WP_DEBUG\', true);
define(\'WP_DEBUG_LOG\', true);
define(\'WP_DEBUG_DISPLAY\', false);
@ini_set(\'display_errors\',0);